Alpha-tantalum on Silicon Achieves 3.77, 4.39 K Critical Temperatures with 7, 10 Nm Beta-Tantalum Underlayer

Tantalum films are emerging as a key material for building the superconducting circuits of tomorrow, promising enhanced performance in quantum technologies, but understanding how to grow high-quality tantalum remains a significant challenge. Researchers led by E. V. Zikiy, N. S. Smirnov, and E. A. Krivko, investigated the growth of tantalum on silicon substrates, revealing a surprising link between the substrate’s properties and the resulting film structure. The team demonstrates that the substrate’s Debye temperature, a measure of its atomic vibrations, plays a crucial role in determining the type of tantalum that forms, challenging previous assumptions about temperature’s influence. This work establishes the necessity of an initial layer of a different tantalum form for successful growth, and ultimately enables the creation of compact tantalum resonators exhibiting exceptionally high performance, with internal quality factors exceeding ten million, a major step towards more powerful and efficient superconducting devices.

This work presents a comprehensive study of alpha-tantalum film growth on various substrates, elucidating the underlying mechanisms governing film formation. The research experimentally confirms that a substrate material’s Debye temperature plays a decisive role in the phase selection during tantalum film growth, challenging prior assumptions regarding the influence of substrate temperature. Crucially, the team confirms that alpha-tantalum growth only commences after the formation of a 7-10 nanometer thick beta-tantalum underlayer, a critical observation for controlling film properties and achieving desired critical temperatures.

Interfacial Thermal Conductance Generalization and β-Ta Structure

Scientists have achieved a significant breakthrough with the successful growth of high-quality alpha-tantalum films directly on silicon substrates, a crucial step towards scalable superconducting quantum computing. This research addresses a long-standing challenge, demonstrating that alpha-tantalum requires the initial formation of a beta-tantalum underlayer before it can effectively grow on silicon. This discovery reveals a critical understanding of the growth mechanism.

Alpha-Tantalum Growth Needs Beta-Tantalum Underlayer

Experiments demonstrate a direct correlation between the substrate’s Debye temperature and the selection of the alpha-tantalum phase, confirming that the initial beta-tantalum layer is not incidental, but a necessary precursor for alpha-tantalum formation. Researchers optimized the sputtering process, achieving ultralow surface roughness and meticulously characterizing the films. The team successfully grew alpha-tantalum films ranging from 20 to 150 nanometers in thickness, achieving critical temperatures ranging from 3. 77 K to 4. 39 K, demonstrating control over the material’s superconducting properties.

This breakthrough delivers compact tantalum coplanar resonators with dimensions of 4/10. 5/4 μm, exhibiting an exceptionally high internal quality factor exceeding 10 million at single-photon excitation powers. This performance surpasses previously demonstrated values for tantalum resonators on sapphire substrates, and the researchers prove that the 10 nm beta-tantalum sublayer does not negatively impact the coherence of superconducting quantum circuits. Data confirms that implementing 2 μm deep silicon trenching further enhances resonator quality, while maintaining the integrity of the substrate-metal interface.

This research is transformative because it enables the use of silicon substrates, essential for large-scale quantum processor fabrication and 3D integration, with high-performance tantalum-based superconducting circuits. The findings show that pre-cleaning of substrate adsorbates is a secondary factor, with the initial beta-tantalum layer being the primary driver of alpha-tantalum growth. This detailed understanding of tantalum film growth and structural properties is of great importance for advancing the development of tantalum-based superconducting quantum circuits and realizing the potential of scalable quantum computing.

Alpha-Tantalum Growth on Silicon Explained

This research establishes a detailed understanding of alpha-tantalum film growth on silicon substrates, a promising material for superconducting quantum circuits. The team demonstrates that achieving high-purity alpha-tantalum requires careful control of substrate temperature and confirms that thermal activation is crucial for its formation, with a critical temperature of 450°C for silicon. Importantly, the study reveals a two-stage growth process, beginning with a beta-tantalum underlayer approximately 7-10 nanometers thick, followed by nucleation of the alpha phase. The findings highlight the significant role of the substrate’s Debye temperature in influencing the phase selection during tantalum film growth; higher Debye temperatures promote faster thermalisation and favour the initial beta-phase formation.

Researchers fabricated compact tantalum resonators on silicon, achieving internal quality factors exceeding 10 million at single-photon excitation powers, significantly outperforming comparable aluminum resonators. While the study confirms the importance of substrate pretreatment for removing surface contaminants, it also acknowledges that this alone cannot induce alpha-tantalum formation without sufficient heating. Future work could investigate the formation mechanism of the nanoridge structures observed on the film surface and explore further optimisation of post-processing treatments to minimise losses at the metal-air interface.